/*

 * Licensed to the Apache Software Foundation (ASF) under one

 * or more contributor license agreements.  See the NOTICE file

 * distributed with this work for additional information

 * regarding copyright ownership.  The ASF licenses this file

 * to you under the Apache License, Version 2.0 (the

 * "License"); you may not use this file except in compliance

 * with the License.  You may obtain a copy of the License at

 *

 *     http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

package com.bff.gaia.unified.sdk.state;



import com.bff.gaia.unified.sdk.annotations.Experimental;

import com.bff.gaia.unified.sdk.annotations.Experimental.Kind;

import com.bff.gaia.unified.sdk.transforms.Combine.CombineFn;



import javax.annotation.Nonnull;



/**

 * A {@link ReadableState} cell defined by a {@link CombineFn}, accepting multiple input values,

 * combining them as specified into accumulators, and producing a single output value.

 *

 * <p>Implementations of this form of state are expected to implement {@link #add} efficiently, not

 * via a sequence of read-modify-write.

 *

 * @param <InputT> the type of values added to the state

 * @param <AccumT> the type of accumulator

 * @param <OutputT> the type of value extracted from the state

 */

@Experimental(Kind.STATE)

public interface CombiningState<InputT, AccumT, OutputT> extends GroupingState<InputT, OutputT> {



  @Override

  @Nonnull

  OutputT read();



  /**

   * Read the merged accumulator for this state cell. It is implied that reading the state involves

   * reading the accumulator, so {@link #readLater} is sufficient to prefetch for this.

   */

  AccumT getAccum();



  /**

   * Add an accumulator to this state cell. Depending on implementation this may immediately merge

   * it with the previous accumulator, or may buffer this accumulator for a future merge.

   */

  void addAccum(AccumT accum);



  /** Merge the given accumulators according to the underlying {@link CombineFn}. */

  AccumT mergeAccumulators(Iterable<AccumT> accumulators);



  @Override

  CombiningState<InputT, AccumT, OutputT> readLater();

}